<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <div>
        数量:
        <input type="text"  id="num" value="1" oninput="changeNum()">
    </div>
    <div>
        单价:
        <input type="text"  id="price" value="1" oninput="changePrice()">
    </div>
    <div>
        总价:  <span id="total"> </span>
    </div>
    <script>
        var sum = document.getElementById("sum")
        var price = document.getElementById("price")
        var total = document.getElementById("total")
        total.innerText = Number(num.value) * Number(price.value)
        function changeNum(){
            var numVal=Number(num.value)
            total.innerText = numVal * Number(price.value)
        }
        function changePrice(){
            var priceVal = Number(price.value)
            total.innerText = priceVal * Number(num.value)
        }

    </script>
</body>
</html>